MMR is a number that shows how 'good' you are. This number matches you with people of the same skill level (at least it's supposed to).
Each league, eg gold iv, bronze i, ect has an average MMR, of all the people who belong in it.
This means that of yoh are in silver 3 but your MMR is equivalent to gold 5, the system will 'asume' you are better than where you belong. So it will reward you with more LP if you win, and it will take away less id you lose.
Of you are in plat 3 and your MMR is around plat 3, the system will reward 'average' and will take away 'average' LP.
If you are in gold 5 but your MMR is for silver 5, the system will rewatd you with very little LP and it will take away alot.
This is how they are connected.
Also you must know that MMR is ONLY, and ONLY affected by wins and losses. This means if you win with 30-2 score it is the same thing as winning with 2-30 score same for losses. Kills, deaths, assists, cs, wards, nothing matters ONLY wins and losses.
Also be aware! I was inder the impression that MMR resets at the beginning of new seasons, but it doesn't !!! They do what is called 'soft reset' meaning that if your MMR is very high, it will get closer to average but not average, if its too low it will get a little higher closer to average but not average.
I don't understand what you mean by placement but i hope i helped for the rest.